The present invention relates to a new and improved construction of smoke detector operating according to the radiation extinction principle, wherein there is detected the radiation attenuation caused by smoke present in a measuring path and there is triggered, at a given radiation attenuation, an alarm signal by means of an evaluation circuit.
With a smoke detector of this type there must be detected a relatively small decrease of the radiation which is directed by a radiation transmitter upon a radiation receiver. In this regard, it is a disadvantage that a similar effect as caused by the presence of smoke in the measuring path equally can be caused, for instance, by aging of the radiation source, dust contamination of optically effective surfaces or the temperature characteristics of the radiation transmitters and receivers. Thus, a spurious alarm signal can be triggered even without the presence of smoke, or else the smoke detector becomes insensitive and thus useless.
According to U.S. Pat. No. 3,994,603, granted Nov. 30, 1976, this shortcoming can be eliminated in that there is provided a comparison radiation beam, which is not or less influenced by smoke. By means of a comparison radiation receiver the evaluation circuit compensates for changes in radiation which are not caused by smoke.
While the aforementioned disadvantages thus can be extensively avoided, it is however not possible to reliably distinguish in this manner smoke from other types of suspended particles, such as dust particles or fog.